Design of ultra-wideband bandstop filter using defected ground structure

A simple wideband bandstop filter which yields equal ripples in the stopband is described. The filter comprises only two resonators and a connecting transmission line. Ultra-wide stopband characteristic is achieved by realising three transmission zeros near the passband edges. Stopband rejection depth and bandwidth can be controlled by impedances of the configuration. The defected ground structure is chosen to realise the high impedance line. To validate the proposed topology, a compact microstrip bandstop filter centred at 1.5 GHz with a measured 20 dB rejection bandwidth of about 139% is fabricated.